1. Field of the Invention
The present invention relates to TV-signal receiving tuners, and more particularly, to a TV-signal receiving tuner for both VHF and UHF frequency bands, which prevents an oscillation signal of a local oscillator in the TV-signal receiving tuner from leaking.
2. Description of the Related Art
FIG. 6 shows a conventional TV-signal receiving tuner. This V-U integrated-type TV-signal receiving tuner is formed by integrating a VHF-band TV-signal receiving tuner and a UHF-band TV-signal receiving tuner into one unit, and includes a VHF-band input terminal 31, a VHF-band high-frequency circuit section 32, a VHF-band frequency converter 33, a UHF-band input terminal 34, a UHF-band high-frequency circuit section 35, a UHF-band frequency converter 36, an intermediate-frequency amplifier 37, a band switcher 38, and an output terminal 39. The VHF-band frequency converter 33 has a VHF-band mixer 40 and a VHF-band local oscillator 41. The UHF-band frequency converter 36 has a UHF-band mixer 42 and a UHF-band local oscillator 43.
The VHF-band high-frequency circuit section 32 includes a high-frequency amplifier, a tuning circuit, and a filter none of them are shown), selects a VHF-band TV signal to be received, and amplifies it.
In the same way, the UHF-band high-frequency circuit section 35 includes a high-frequency amplifier, a tuning circuit, and a filter (none of them are shown), selects a UHF-band TV signal to be received, and amplifies it.
The band switcher 38 is connected to the VHF-band frequency converter 33 and to the UHF-band frequency converter 36, and supplies a switching voltage to one of the VHF-band frequency converter 33 and the UHF-band frequency converter 36 according to the band to be received. When a VHF-band TV signal is to be received, the switching voltage is generated only at a first control terminal 44, the switching voltage is supplied to the VHF-band frequency converter 33, the VHF-band frequency converter 33 is set to an operating state, and the UHF-band frequency converter 36 is set to a not-operating state. When a UHF-band TV signal is to be received, the switching voltage is generated only at a second control terminal 45, the switching voltage is supplied to the UHF-band frequency converter 36, the UHF-band frequency converter 36 is set to an operating state, and the VHF-band frequency converter 33 is set to a not-operating state.
When a VHF-band TV signal is received, the VHF-band TV signal received at an antenna (not shown) passes through the VHF-band high-frequency circuit section 32, is frequency-converted by the VHF-band mixer 40 and the VHF-band local oscillator 41 in the VHF-band frequency converter 33, is amplified by the intermediate-frequency amplifier 37 as an intermediate-frequency signal IF, and is output from the output terminal 39. The intermediate-frequency signal IF output from the output terminal 39 is detected by a detection circuit (not shown).
When a UHF-band TV signal is received, the UHF-band TV signal received at an antenna (not shown) passes through the UHF-band high-frequency circuit section 35, is frequency-converted by the UHF-band mixer 42 and the UHF-band local oscillator 43 in the UHF-band frequency converter 36, is amplified by the intermediate-frequency amplifier 37 as an intermediate-frequency signal IF, and is output from the output terminal 39. The intermediate-frequency signal IF output from the output terminal 39 is detected by a detection circuit (not shown).
In the above conventional V-U integrated-type TV-signal receiving tuner, since the distances between the circuits are short, isolation between the VHF-band frequency converter 33 and the UHF-band frequency converter 36 cannot be sufficiently provided. Therefore, an oscillation signal generated by the local oscillator of an operating frequency converter leaks to the not-operating frequency converter, and is emitted from the antenna to the air to give disturbance to other receivers.
When a VHF-band TV signal is received, an oscillation signal generated by the VHF-band local oscillator 41 leaks to the UHF-band frequency converter 36, and is emitted to the air through the UHF-band high-frequency circuit section 35 and the antenna. When a UHF-band TV signal is received, an oscillation signal generated by the UHF-band local oscillator 43 leaks to the VHF-band frequency converter 33, and is emitted to the air through the VHF-band high-frequency circuit section 32 and the antenna.
Accordingly, it is an object of the present invention to provide a TV-signal receiving tuner which prevents a case in which an oscillation signal generated by an operating local oscillator leaks to an not-operating frequency converter and is emitted to the air through an antenna as extraneous emission to give disturbance to other receivers.
The foregoing object is achieved by the present invention through the provision of a TV-signal receiving tuner for both VHF and UHF frequency bands, including a VHF-band frequency converter having a VHF-band mixer and a VHF-band local oscillator, for converting a VHF-band TV signal to a signal having an intermediate frequency; a UHF-band frequency converter having a UHF-band mixer and a UHF-band local oscillator, for converting a UHF-band TV signal to a signal having an intermediate frequency; and a band switcher for selectively operating the VHF-band frequency converter or the UHF-band frequency converter, wherein a switch circuit is provided for at least one of a previous stage of the VHF-band frequency converter and a previous stage of the UHF-band frequency converter; and the switch circuit passes a high-frequency signal when a frequency converter at a subsequent stage of the switch circuit is operating, and does not pass a high-frequency signal when the frequency converter at the subsequent stage of the switch circuit is not operating.
Therefore, even if an oscillation signal generated by the local oscillator of the operating frequency converter leaks to the not-operating frequency converter, the oscillation signal is not emitted from the antenna to the air. Extraneous emission is suppressed.
The TV-signal receiving tuner may be formed such that a VHF-band high-frequency circuit section is provided at a previous stage of the VHF-band frequency converter; a UHF-band high-frequency circuit section is provided at a previous stage of the UHF-band frequency converter; and the switch circuit is provided at at least one of a location between the VHF-band high-frequency circuit section and the VHF-band frequency converter, and a location between the UHF-band high-frequency circuit section and the UHF-band frequency converter.
In this case, since the oscillation signal which causes extraneous emission can be blocked near the emission source, extraneous emission is effectively reduced.
The TV-signal receiving tuner may be formed such that the switch circuit is provided at at least one of a location immediately before the VHF-band frequency converter and a location immediately before the UHF-band frequency converter.
In this case, the switch circuit can be combined with the frequency converter and other circuits into an integrated circuit to make the circuit compact.
The TV-signal receiving tuner may be formed such that the switch circuit includes a transmission line to which an input end and an output end of the switch circuit are connected, and a transistor or a diode for making and breaking a connection between the transmission line and the ground is provided.
The TV-signal receiving tuner may be formed such that the switch circuit is provided with a transistor or a diode for making and breaking a connection between an input end and an output end of the switch circuit.
In this case, since the switch circuit is formed of a transistor or a diode, the switch circuit is made compact and extraneous emission is suppressed without increasing the volume of the tuner.
The TV-signal receiving tuner may be formed such that the switch circuit includes a transmission line to which an input end and an output end of the switch circuit are connected; a first transistor or a first diode for making and breaking a connection between the transmission line and the ground is provided; a second transistor or a second diode for making and breaking a connection between the input end and the output end is provided; and when the connection between the transmission line and the ground is broken, the connection between the input end and the output end is made, and when the connection between the transmission line and the ground is made, the connection between the input end and the output end is broken.
In this case, since the transmission line of the switch circuit can be grounded and electrically disconnected at the same time, extraneous emission is further suppressed.